Introduction

In a Varian accelerator, the upper jaws and the lower jaws define a rectangular field. Within this rectangular field, the multileaf collimator (MLC) is used to adjust the field to the shape of the target. The MLC radiation transmission values can be 0·90–4·40% higher for fields only covered by MLC than for those shielded by both MLC and Jaws using a 6 MV beam. MLC transmission values of 1·8% have been reported for a 10 MV beam. ^{Reference Mohan, Jayesh and Joshi1–Reference Schmidhalter, Fix and Niederer3} The largest amount of transmitted radiation has been measured at the leaf tip and between two adjacent leaves.

The dynamic jaw tracking technique (JTT) keeps the rectangular field of the jaws as tight as possible to the actual aperture of the MLC. This arrangement decreases the undesired doses delivered to normal tissue and organs at risk (OARs), compared to the arrangements in the static jaw technique (SJT). ^{Reference Schmidhalter, Fix and Niederer3–Reference Pokhrel, Sanford and Halfman16}

Several authors have explored the patient protection potential improvements in intensity-modulated radiotherapy (IMRT) with JTT. Schmidthalter et al. analysed this issue in three academic and two clinical IMRT cases. The academic cases showed reductions of local unwanted dose up to 50% depending on the measurement conditions. Depending on the localisation, dose decreases between 1·5 and 7% were achieved outside the planning target volumes (PTVs) in the clinical cases when using JTT. The total number of applied monitor unit (MU) increased by about 3%. ^{Reference Schmidhalter, Fix and Niederer3}

Joy et al. took static IMRT beams and adjusted the jaws to a margin outside each MLC aperture to create JTT plans in a sample of clinical cases. Because of the decrease in target dose, these JTT plans had to be renormalised. Most patients had less than 2% improvements in the normal tissue volumes receiving a dose of 5, 10 and 20 Gy (V5, V10 and V20). The total number of MUs and maximum doses (Dmax) resulted in higher values than those of the static plans. These authors could not find any parameters that clearly identified which patients would benefit most from JTT. ^{Reference Joy, Starkschall and Kry4}

Chen et al. studied the effect of adjusting the position of the jaws on IMRT to better shield OARs during treatments of gynaecological patients. This resulted in better conformity index (CI) and OARs protection. In addition, a larger number of MUs per plan was registered. ^{Reference Chen, Chen and Huang5} Feng et al. compared JTT and SJT in a sample of twenty-eight patients. They found that JTT plans decreased the dose to the whole body, as evaluated using V5, V10, V20 and mean dose (Dmean) values, between 2·6 and 0·4%, compared to SJT plans. The dose reductions for OARs ranged from 2·2 to 28·6%, more obvious for structures of small volumes or those far from the targets. ^{Reference Feng, Wu and Zhang6} Shi et al. studied thirty-two patients with oesophageal cancer. JTT resulted in reductions between 8 and 3% in V5-lung, between 2·5 and 1% in V20-lung, and between 7 and 3% in mean dose, compared to SJT, without significant differences in PTV coverage and conformity. For total MUs, JTT plans resulted in increases between 6 and 7% compared to SJT plans. ^{Reference Shi, Lai and Chen7}

JTT can also be applied to volumetric modulated arc therapy (VMAT) treatments. Kim et al. compared JTT plans and SJT plans in terms of reductions in jaw size amounts and OARs doses. They observed a jaw size reduction more noticeable in the X direction than in the Y direction, with dose reductions ranging between 1 and 12% in JTT plans. ^{Reference Kim, Park and Park8} Snyder et al. found that JTT decreased spinal cord dose in IMRT treatments more effectively than in VMAT treatments. ^{Reference Snyder, Wen and Huan9} Park et al. treated 31 VMAT cases with liver, pancreatic and lung cancer. They calculated all plans under jaw tracking and fixed jaw conditions and concluded that the dose to OARs and the low-dose volumes can be significantly reduced by the application of JTT. ^{Reference Park, Cho and Kim10} They also found that the difference in low-dose volumes between JTT and SJT increased with the difference between effective maximum field size and the equivalent spherical diameter of the PTVs.

In another study by Wu et al., the mean dose reduction to various OARs ranged from 0·1 to 7·8%, without sacrificing the target dose coverage. ^{Reference Wu, Jiang and Yue11} Yao et al. studied a sample of patients with nasopharyngeal carcinoma (NPC) and single brain metastasis. They found dosimetry differences favouring JTT plans. The OARs doses were reduced from 1 to 19%. The number of MUs in JTT plans increased slightly compared to those in SJT plans. They noticed that the benefits of JTT plans were greater for NPC patients with larger tumour volumes and treatment fields. ^{Reference Yao, Zhang and Chen12} Raj Mani et al. estimated the influence of JTT in head and neck cancer patients compared to the static technique. The dose reductions with JTT in V5, V10, V20, V30 and Dmean ranged from 2 to 5% in IMRT plans and from 1 to 3% in VMAT plans. ^{Reference Mani, Upadhayay and Das13} Thongsawad et al. evaluated dose reduction with JTT in VMAT plans for patients with cancer in various localisations. They found integral-dose reductions in the V5 region and in other sensitive organs. ^{Reference Thongsawad, Khamfongkhruea and Tannanonta14} Besides, they considered the jaw tracking distance and concluded that the greater this value, the greater the dose reduction.

In another study, Murakami et al. assessed OARs dose reduction with JTT in VMAT treatments. They found a statistically significant decrease in OARs doses, with a maximum of 1·2%. They proposed a metric to quantify jaw movements and found significant correlations between this metric and the reduction of the values of some dosimetric parameters of the OARs. ^{Reference Murakami, Magome and Matsubayashi15} Pokhrel et al. estimated the reduction of the dose to healthy tissue in the treatment of single-isocentre/two-lesion lung stereotactic body radiotherapy with JTT-VMAT. Target coverage was similar for SJT and JTT plans. Normal lung doses were reduced by 8–11% with JTT and increased with the distance of the tumours from the isocentre. ^{Reference Pokhrel, Sanford and Halfman16}

In most of the previous reports, the location of the isocentre is not generally specified. The most common arrangement in VMAT locates the isocentre at or near the target centre. However, in the case of peripheral targets, placing the isocentre at the patient’s midline can be advantageous. This arrangement diminishes clearance issues, spare couch motions and allows plans with one or more full-arc rotations. ^{Reference Jeevanandam, Agnew and Irvine17–Reference Ross, Kim and Chen22} Regarding the beam field-of-view, the relative target position in this condition describes a wider oscillation than that with a more centred target. Thus, the MLC leaves are forced to move in wider ranges, and the subsequent secondary collimation will encompass a wider area, assuming a static arrangement. The area blocked only by the MLC tertiary collimation will then increase, with the corresponding rise in radiation leakage. In such conditions, JTT could achieve even more important radiation leak reductions (Figure 1).

Figure 1. Representation of a SJT plan (top) and a JTT plan (bottom) for a patient. Axial dose representation (left) and beam eye’s views for an illustrative gantry angle and arc (right).

The objective of the present work was to explore possible dosimetry differences between JTT plans and SJT plans. As a novelty, it focused on patients with peripheral targets and the plan isocentre was located at patient’s midline. Possible correlations between the reduction of the jaw secondary collimation with JTT compared to SJT and the potential dosimetry benefits of the technique were also explored.